The present invention relates generally to the field of interferometric sensing apparatus and, more particularly, to apparatus for preventing or reducing polarization fading in sensing interferometers.
The present invention is particularly adapted for use in a seismic array which includes fiber optic interferometric sensing apparatus. Typically such sensing apparatus uses one of a number of typical means for varying the optical path length in an optical fiber which is used as the sensing means to detect acoustic signals well known in seismic exploration techniques.
In the seismic exploration art, there is a continuing need to improve the rate of successfully locating recoverable reserves of hydrocarbons. Advancements in seismic sensing equipment have a direct impact on the rate of such success. Known seismic sensing equipment includes large arrays of optical hydrophones, typically using interferometric sensing techniques.
However, such systems are prone to polarization fading. Polarization fading is due primarily to changes in the mechanical configuration of the sensing fibers, which changes result primarily from the effects of strain, temperature, and other effects. These changes cause random changes in the plane of polarization of the propagating beams, thus causing polarization fading.
Thus, polarization fading is a known phenomenon in such interferometric sensing systems. In these systems, a light beam is split by a beam splitter into two beams, and then recombined after the two beams have followed different paths. In the following explanation of polarization fading, assume that the z axis represents the direction of propagation of the light, and that the x and y axes lie in the plane orthogonal to the direction of propagation.
A monochromatic beam is incident on the beam splitter. The electric vector of the incident beam is given by
Ex=2 cos xcfx89t Ey=0 
The electric vectors of the emerging beams are given by
Ex=cos(xcfx89t+xcex8) Ey=0 
Ex=cos(xcfx89t+xcfx86) Ey=0 
where xcex8, xcfx86 are the path lengths of the two arms of the interferometer, expressed in radians. The two beams interfere when recombined and the light intensity is given by the time average of
I=(xcexa3Ex)2+(xcexa3Ey)2 
For this case,
 less than I greater than =1+cos(xcex8xe2x88x92xcfx86) 
If the plane of polarization of the second beam is rotated through 90xc2x0 with respect to the first beam as it traverses the second arm, then
Ex=0 Ey=cos(xcfx89t+xcfx86) 
and
 less than I greater than =1 
Thus, the term cos(xcex8xe2x88x92xcfx86) is suppressed, resulting in polarization fading.
More particularly, any non-circularity in the optical fiber, or any stress or torsion applied to the fiber, induces birefringence. A single mode optical fiber then supports two modes which propagate with different phase velocities. The polarization state therefore evolves along the optical fiber. The birefringence can be linear (induced by stress or non-circularity) or circular (induced by torsion), or a combination of linear and circular. Changes in temperature or the physical disposition of the fiber will affect the linear and circular birefringence differently, thus causing the state of polarization at the output of the optical fiber to fluctuate.
Polarization fading is a phenomenon that occurs with linearly polarized beams whose planes of polarization are orthogonal. Circularly polarized beams do not exhibit polarization fading because a polarizer will always transmit a circularly polarized beams as a linearly polarized beam with a plane of polarization determined by the orientation of the polarizer and with an intensity of 50%. Consequently, any two circularly polarized beams passing through a polarizer emerge with the same plane of polarization and can interfere. For this reason, it is only necessary to consider states of polarization that lie on the equator of the Poincare sphere.
These effects are important in an optical fiber interferometric sensor because the light from one arm of an interferometer will not interfere with light from another arm of the interferometer if the two states of polarization are orthogonal. This condition is generally known as polarization fading, i.e. the visibility of the interference fringes fades to zero as the states of polarization of the two polarized modes become orthogonal.
A number of systems have been proposed to reduce or eliminate polarization fading. One such system is shown in U.S. Pat. No. 5,173,743 to Kim. In the system of Kim, an extended interferometer having a plurality of sensors and a compensating interferometer are used. They are driven from a pulsed optical signal source wherein the optical signal comprises sequences of two pulses each. To prevent polarization fading, the polarization of a predetermined one of each two-pulse sequence is switched, preferably orthogonally, from sequence to sequence. Interference pulse output groups are produced for each two-pulse driving sequence. Each output group has the same number of usable pulses as the number of sensors in the interferometer. In Kim, the return optical signal is first applied to the sensor which converts the optical signal into an electrical signal. Then, a switch is connected to receive the electrical signals from the sensor. A timer delivers a timing signal to the switch to cause incoming pulses to be distributed alternately to another set of switches. The timer sends timing signals to this set of switches to cause them to switch consecutively from one output to the next. Such a system significantly adds to the cost and complexity of the sensor array.
Another known system is referred to as the Litton tri-cell. The Litton tri-cell takes advantage of the fact that if two beams are incident on a polarizer whose plane of polarization is not at right angles to either of the beams, then the beams emerge from the polarizer with attenuated amplitudes but with the same plane of polarization, so interference can take place. However, the Litton tri-cell requires a minimum of three polarizers and three detectors to ensure that at least one of the polarizers is not at right angles to either beam.
Another known system is referred to as a polarization-maintaining fiber system. That system uses fiber that maintains the state of polarization (SOP) along the fiber, and thus does not have a problem with orthogonal beams. However, the use of such fiber is uneconomical, and also requires polarization-maintaining connectors and splices, which are impractical.
Thus, there remains a need for a system having an array of interferometric sensors which is simple, practical, economic, and effective in eliminating polarization fading. The present invention is directed to such a system.
As previously described, polarization fading occurs when the two light beams arriving at a detector are at right angles to one another. If the beams are orthogonal, they do not create an interference pattern, and the signal of interest is lost.
The present invention provides a scrambler in front of a polarizer, followed by the detector. In that way, although the two beams remain orthogonal to each other, they are continuously rotated, relative to the polarizer. In some positions, both beams pass through the polarizer and interfere, thus eliminating polarization fading. The signal is amplitude modulated at the rotation frequency of the scrambler, but this modulation is removed by low pass filtering.
The system ensures equal excitation of all states of linear polarization and thus gives 50% visibility of the interference signal at all times. As used herein, the term xe2x80x9cscramblerxe2x80x9d refers to a device that takes an arbitrary input state of polarization and rotates it continuously about the equatorial axis of the Poincare sphere. A scrambler is typically a device that employs the electro-optic effect in a lithium niobate crystal. The input beam is split into two orthogonal linear polarizations, corresponding to the two propagation modes in the crystal (the TE and TM modes of a dielectric wave guide).
The application of a voltage gives rise to two effects. The first of these effects is retardation. The phase difference between the two modes, at the output, is linearly proportional to the applied field. This allows the SOP to be changed between linear and circular polarization.
The second effect is mode conversion. In the presence of an electric field some of the energy in the TM mode is converted to the TE mode. This allows the plane of polarization to be rotated through an angle, proportional to the applied field. The effect also depends on the relative phase of the two modes.
The effect can be combined in integrated optic devices to produce scramblers and controllers of the type just described. Thus, the present invention provides a practical solution to polarization fading, and is adaptable to many applications, such as for example multiplexed arrays.